An optical fiber propagates light by total internal reflection. A light beam introduced into one end of the optical fiber is propagated essentially without any loss of energy to the other end of the optical fiber, even though the optical fiber may be quite long. This property of the optical fiber is utilized in light-based communication systems in which information is encoded onto a light beam, introduced into the optical fiber, propagated through the optical fiber, received at the other end of the optical fiber, and processed to recover the information. Because of its high frequency, the light beam may carry a great deal of information.
The capacity of the optical fiber to carry information may be further increased by simultaneously transmitting a number of light beams through the optical fibers. For example, a first light beam of a first wavelength (i.e., a first channel) may carry information for a first user, a second light beam of a second wavelength (i.e., a second channel) may carry information for a second user, and so on. The single optical fiber may carry a large number of light beams.
In one approach, dense wavelength division multiplexing (DWDM), the light beams of slightly different wavelengths are multiplexed together, transmitted through the optical fiber, and de-multiplexed at the receiving end to separate the various light beams. There is a relatively limited total wavelength range that may be used, due to other technical limitations. For many applications, the wavelengths must lie in a specific narrow range, as for example between 1528 and 1565 nanometers. The practical limit for the number of light beams that may be transmitted at once within such a total wavelength range is the ability to isolate the light beams and filter the light beams to ensure wavelength separation between the light beams. If too many light beams are transmitted at once so that their wavelengths are too close together, it will not be possible to isolate them in a wavelength sense, and there will be cross talk and interference between the light beams.
Filters have been developed to wavelength-isolate the light beams. Such filters are placed into the light beam to pass only a selected wavelength of light. Examples of such filters are tunable etalon filters and graded thin-film coatings. These filters are operable and are widely used, but they have shortcomings in their ability to isolate the various wavelengths, particularly as the wavelengths are made ever-closer together as the density of communications is increased. There is accordingly a need for an improved approach to light filtering, particularly in DWDM and other types of optical fiber communications systems. The present invention fulfills this need, and further provides related advantages.